1. Field of the Invention
This invention relates to a digital rotation detecting apparatus for detecting a rotation angle and the rotational speed of a rotation body such as a motor.
2. Description of the Prior Art
FIG. 1 is a block diagram of the conventional digital rotation detecting apparatus disclosed in the 546 National Convention of Japanese Institute of Electric Engineers in 1982. In FIG. 1, reference numeral 1 designates a two-phase resolver which rotates in association with a rotary body, such as a motor, 2 designates a reference oscillator circuit of the fixed frequency having a two-phase output, 3 designates a phase difference detector for detecting a phase difference between the two-phase output of the reference oscillator circuit 2 and that of the two-phase resolver 1, 4 designates a loop filter for proportional-plus-integral-computing the aforesaid phase difference, 5 designates a voltage control oscillator (to be called the VCO hereinafter) for generating pulses of the frequency corresponding to the output of the loop filter 4, 6 designates a counter which counts the output pulse of the VCO 5, 7 designates an exciter which generates the two-phase sine-wave output of the phase value corresponding to the counted value by the counter 6 so as to excite the two-phase resolver 1, and 8 designates a latch circuit for latching the counted value by the counter 6 by means of the pulse generated by the reference oscillator circuit 2.
Next, explanation will be given on operation of the aforesaid conventional apparatus. At first, when a phase angle of the two-phase resolver 1 is represented by .theta..sub.r and an excitation phase of the exciter 7 by .theta..sub.o, the two-phase resolver 1 generates the two-phase sine wave of phase angle (.theta..sub.o -.theta..sub.r), at which time when the phase of two-phase output of the reference oscillator circuit 2 is represented by .theta.*, an output .DELTA..theta. of the phase difference detector 3 is given in the following equation: EQU .DELTA..theta.=.theta.*-(.theta..sub.o -.theta..sub.r) (1),
where since the portion surrounded with the broken line 10 in FIG. 1 constitutes a closed loop of phase synchronization generally called the PLL (Phase Locked Loop), automatic control is carried out to obtain .DELTA..theta.=0. Hence, the following equation (2) is derived from the equation (1): EQU .theta.*=.theta..sub.o -.theta..sub.r ( 2).
In the equation (2), when .theta.*=0, .theta..sub.o =.theta..sub.r is obtained, in other words, when the value of .theta..sub.o is sampled at the time of .theta.*=0, the samples value indicates .theta..sub.r. Also, since the counted value by the counter 6 is proportional to the excitation phase .theta..sub.o, when the latch circuit 8 latches the counted value by the counter 6 at the timing of outputting the pulse by the reference oscillator circuit 2 at every cycle of .theta.*=0 thereof, the phase angle .theta..sub.r of the two-phase resolver 1 at the time of .theta.*=0 is to be detected. When a difference .theta..sub.r (NEW)-.theta..sub.r (OLD) between the phase angles .theta..sub.r twice sampled is divided by a cycle period T* of the phase .theta.*, the rotational speed is given in the following equation: ##EQU1##
The conventional digital rotation detecting apparatus constructed as the above-mentioned samples the phase angle .theta..sub.r at the time when the phase of the two-phase output of the reference oscillator circuit is zero, that is, when .theta.*=0, whereby .theta..sub.r at a desired time cannot be obtained, and a detection delay of T* at a maximum is created. For a rotation control apparatus for a motor using a microcomputer, the timing of introducing into the control apparatus the detected phase angle .theta..sub.r and the cycle period thereof are often not-identical with the timing of .theta.*=0 or the cycle period of .theta.*. Therefore, the detection delay is also created to enlarge a detection error in the last phase angle .theta..sub.r. As a result, the problem has been created in that the speed computation executed by the microcomputer and given in the equation (3) of course causes the error.
In order to improve the detection accuracy for the phase angle .theta..sub.r, the maximum counted value by the counter 6 need only be larger, but it is required therefor either to raise the oscillation frequency of the VCO 5, or to lower the reference oscillation frequency f*, or to perform both the processings. The maximum oscillation frequency of VCO 5, however, has the upper limit, so that it is necessary to lower the reference oscillation frequency f* to obtain a high accuracy for detecting the phase angle .theta..sub.r.
Now, the two-phase resolver 1 is excited with the frequency of f.sub.o =f*+f.sub.r, but when the resolver 1 reversely rotates, the frequency becomes f.sub.o =f*-.vertline.f.sub.r .vertline.. Accordingly, the excitation frequency f.sub.o approaches zero as the reference oscillation frequency f* is reduced to approach .vertline.f.sub.r .vertline., thereby creating the problem in that the resolver 1 must be considered to be low-frequency-excited and the exciter 7 also must take a large current capacity.